For a positive constant $c,$ in spherical coordinates $(\rho,\theta,\phi),$ find the shape described by the equation
\[\rho = c.\](A) Line
(B) Circle
(C) Plane
(D) Sphere
(E) Cylinder
(F) Cone

Enter the letter of the correct option.
Explanation: In spherical coordinates, $\rho$ is the distance from a point to the origin.  So if this distance is fixed, then we obtain a sphere.  The answer is $\boxed{\text{(D)}}.$

[asy]
import three;
import solids;

size(180);
currentprojection = perspective(6,3,2);

currentlight = (1,0,1);

draw((-1,0,0)--(-2,0,0));
draw((0,-1,0)--(0,-2,0));
draw((0,0,-1)--(0,0,-2));
draw((1,0,0)--(2,0,0));
draw((0,1,0)--(0,2,0));
draw((0,0,1)--(0,0,2));
draw(surface(sphere(1)),gray(0.8));

label("$\rho = c$", (1,1.2,-0.6));
[/asy]